Photosynthetic efficiency is the fraction of light energy converted into chemical energy during photosynthesis in plants. Combining energy from photosynthesis with carbon dioxide, water, and various minerals, plants can form a vast array of compounds.
For fully absorbed sunlight in the 45% of the light that is in the photosynthetically active wavelength range, the theoretical maximum efficiency of solar energy conversion is approximately 11%. Plants, however, do not absorb all incoming sunlight and do not convert all harvested energy into biomass, which brings the theoretical maximum value to around 5% or less (Renewable biological systems for alternative unsustainable energy production (http://www.fao.org/docrep/w7241e/w7241e05.htm, section 1.2.1). FAO Agricultural Services Bulletin (1997).
Even the 45% of light that can be used by plants is not utilized with equal efficiency. Only the blue and red bands of light are used with high efficiency. Bands in between are largely unused. So the actual theoretical photo efficiency of plants is around 1-2% and, in real measurements, a factor of 10 lower for some plants (http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/ligabs.html). These values do not account for the energy plants often dissipate to compensate for heating effects of light they cannot use.
There is a need to provide efficient and cost effective ways to grow and prepare food and to provide light and warmth in remote locations or in power down situations.
It is desirable to supplement sunlight for growing vegetables in higher latitudes and to increase growing seasons. High Intensity Discharge (HID) lighting systems, such as Metal Halide, Ceramic Metal Halide or High-Pressure Sodium, are costly and consume large amounts of power. They also do not deliver bands of light that plants can most efficiently use, and they force plants to waste their metabolic products to shed the unusable light and heat.
There exist manufacturers of light emitting diodes (LED) grow lights for commercial and household use where a grid is up and line power is available. Many companies also manufacture photovoltaic (PV) panels or thermoelectric (TE) appliances to supply supplemental or replacement power for remote or power down situations.
Typically, high direct current voltage from the PV panel is fed into an inverter to produce 120 volts of alternating current (AC). Producers of LED grow lamps typically supply them to operate from AC line power which is then converted to LED voltages or to operate from a regulated DC voltage supply. PV panels and thermoelectric appliances, on the other hand, are usually designed to operate at full power and load at 12-volts DC but may produce over 20-volts DC when not fully loaded. Unregulated, 20-volts would destroy an LED array or series designed for 12-volts. Regulators shed and waste excess voltage and power as heat, and inverters and converters are inefficient, also wasting power as heat.
U.S. Pat. No. 9,200,770 B2 to Chun, which is incorporated herein in its entirety by reference, discloses a solar light using PV panels, batteries and LEDs for use in power down situations and in remote locations. The voltage is regulated and controlled, and the light sources, including LEDs, are selected to produce broad band white light suitable for human vision but not ideally suitable for plants.
It would be desirable and useful to power LED series and arrays directly to produce the narrow bands of light that are optimal for plants. It would be desirable to power LEDs that are optimized to provide warmth and comfort to people in remote or grid down circumstances.